Sedimentary Geology Of The Upper Triassic Flysch In The Eastern Himalaya Orogen, Tibet, China

The Upper Triassic flysch——Langjiexue Group (comprising the "Shannan Terrane") lies in the eastern Himalayan Orogen, southern Tibet. It was tectonically assigned to the northern Tethyan Himalaya and has been identified as a part of the deep-sea sediments on the northern Indian passive margin, or as an accretionary prism with the Xiukang Group within the melange belt of the Yarlung Zangbo Suture Zone (YZSZ). Obviously, the different classification of tectonic property not only may cause the misunderstanding of provenance and basin for the flysch, but also may lead to wrong reconstruction of the Late Triassic paleogeography, paleotectonics and their evolution for the eastern Tethys, such as the Cimmeride, Neotethys, and even northern Gondwanaland.Recently, many attentions have been paid to the Langjiexue Group. Some important scientific issues remain open despite of many achievements or have not been reached an agreement. These issues are mainly related to the lithofacies, depositional system (deposystem), provenance, basin type, paleogeography, and evolution of paleotectonics.To improve and solve some of the issues, lots of works were carried in the entire region of outcrops. In this study, thirty-two lithostratigraphic cross-sections within seventeen geological routes were observed and described, within which stratigraphic thicknesses of different lithologies were measured and calculated at 72 observation stops, over 90 sets (-1000 flowing directions) of paleocurrent indicators were measured, and more than 30 and-60 sandstone samples were collected for analyses of clastic composition and heavy mineral assemblage, respectively. In laboratory, over 1600 detrital zircons of 20 sandstone samples and-80 zircons of six diabase samples were used for analyses of U-Pb isotope dating, over 170 detrital zircons (dated as 400-200 Ma) were chosen for Hf isotope measurement, and over 140 detrital spinels were selected for probe analysis. On the bases of above numerous data, the author integrated the sediment dispersal pattern, microfacies, facies, deposystem and provenance to tentatively probe the basin property, paleogeography, and tectonic evolution. The preliminary results and understandings are summarized as below:Results of the heavy mineral index ZTR values, S/M (sandstone/mudrock) ratios and paleocurrent directions indicate the sediments were transported from north to south. Of them, the mean paleocurrent flowing direction is revised as 185° associated with several groups of southwestward/southeastward transported direction. The composition of the above data together delineates a radial-curved sediment dispersal pattern for the Langjiexue Group. This pattern implies that sediments were transported by feeder channels towards the basin floor where they fanned out in the mid-fan and outer-fan areas by distributary channels, and the channel systems were well developed on the submarine fan.Four facies, channel (A), lobe (B), levee-interchannel (C), and basin plain (D) with nine subfacies (A1-3, B1-3,and C1-3), were distinguished from lithofacies analysis. They comprise six facies associations:channel-interchannel, overbank-levee, crevasse-splay, outer fan-lobe, fan-fringe, and basin plain. Taking into account the facies analysis, sediment dispersal pattern, and restoration of primary stratal width, this work proposed that the Langjiexue Group is consisted of a submarine fan-dominated deep sea deposystem without the preservation of inner fan sub-deposystem, measuring about 400-500 km 600-700 km in size or even more, and it is one of the largest pre-Cenozoic submarine fans ever reported. Four fans could have coalesced laterally, and at least six submarine fan developments were vertically succeeded by mid to outer-fan deposits with progradational to retrogradational successions, indicating the control of tectonic/thermal subsidence and/or rising sea-level. According to the range of sandstone content, the fan is of mud-and sand-rich deposystem, suggesting a medium-far (up to 400-600 km) transport of sediments from the source area.The provenance analysis proved different lithologies and tectonic backgrounds in the source area for the Langjiexue Group. The observed heavy mineral assemblages suggest lithologies of parent areas were sedimentary rocks, granitoids, and metamorphic rocks; of the heavy minerals, pyroxenes and chrome spinels point to basic and ultrabasic sources; RuZi ratios in the west and east are different, implying at least two different sources. The components of studied sandstones fall in the "recycled" and "mixed-arc orogen" fields in the Dickinson triplots. Numerous Cr-spinels and geochemical data from the Shannan Terrane denote several possible parent lithologies:basalts, peridotites, and boninites, likely deriving from oceanic (island) arc/seamount and mid-ocean ridge areas. Those above demonstrated the Langjiexue Group has multiple sources.The multiple sources are also verified by two different populations of U-Pb isotope ages of detrital zircons. They are 600-460 Ma (peak ca.520 Ma) and 260-200 Ma (peak ca. 240 Ma), respectively. More importantly, the former one is originated from the Panafrican orogen, implying one or two or more sources from terranes of the Lhasa Terrane, West Australia, Qiangtang Terrane, and other Gondwanaland. The latter one strongly supports the viewpoint of the Shannan Terrane disaffinity to the Tethyan Himalaya. The combination of the featrured detrital zircon age population of 260-200 Ma (peak ca.240 Ma), much more positive sHf(t) values of detrital zircons dated as-400-290 Ma in the Shannan Terrane than in the Lhasa Terrane, and the existence of Cr-spinel within the Shannan Terrane, illustrates that these two terranes have different sources, or at least, the Lhasa Terrane is not the only source for the Shannan Basin.The multiple sources, different tectonic settings of source areas, and the Early Cretaceous (-140 Ma to 128 Ma) diabase dykes within the Upper Triassic flysch defined to the Comei-Bunbury Large Igneous Province, together suggested that the Langjiexue Group could have been accumulated in a remnant basin in the northern Gondwanaland, which was probably located between the eastern India and western Australia. For this model, source areas are postulated and presumed as the Lhasa Terrane, magmatic arc in its north and back arc in south, seamounts and/or mid-ocean ridges in the south of Neotethys, and the western Australia and eastern India may provide little sediments or they are the supplier of most sediment related to the Panafrican orogeny.Based on the models of basin and paleography, it is speculated:the Shannan Basin did not stop receiving sediments until the terminal Triassic, at the time the Lhasa Terrane northward drifted by the back-arc extension and accretion of mid-ocean ridge; after the separating of the Lhasa from the Australia, the Shannan Terrane could have been loaded onto the northeastern Greater India; during the Jurassic through the early Early Cretaceous, it might have kept its position at the same place and either being the basin basement of coeval Tethyan Himalaya or not; since then, upheaved onto the Greater India, it may have been northward drifting to the collision zone of India and Asia by the rifting and opening of Indian Ocean; and it would have not stopped to be uplifted, deformed and metamorphosed till present. This paleography and tectonic evolution imply that the Langjiexue Group does not belong to the Jurassic-Cretaceous accretionary prism within the YZSZ.